The present invention relates generally to secure fiber optic communications and more particularly to an apparatus which effects the efficient coupling of light from multiple optic fibers into a single optic fiber.
Optical fibers are essentially hollow tubes made of transparent dielectric materials. An annular cladding, or jacket, surrounds a central region, referred to as the core. In step-index fibers, the core and the jacket have fixed refractive indexes: the index of the cladding is less than that of the core. Light rays projected into the fiber are guided along it because of repeated total internal reflections at the cylindrical boundary between the core and the cladding materials. Optical fibers developed within the last five years have removed several impediments to reliable long distance high data rate transmission using devices gated in the time domain. These impediments were chiefly factors causing high attenuation, or light loss (large db/km) afforded to wavelengths available from devices capable of being modulated at the data rates desired (Gigabit/sec rates). However, despite the removal of these impediments, fibers are easily tapped and are seriously underutilized with respect to the available bandwidth.
Fundamental physical problems also suffered by fibers include dispersion, coupling losses, and reflection and refraction. Dispersion is the distortion of pulse shape with distance observed in fiber optics. It is caused by finite differences in the total path traveled both geometrically (intermodal) and through the variations in refractive index with wavelength characteristics of fiber materials (intramodal). Intermodal dispersion may be minimized by control of optical power launch conditions, reflections at interfaces, and large fiber bend radii. Intramodal dispersion may be minimized through use of narrow bandwidth sources, slower data rates, and/or de-skewing electronics at the link ends.
Inefficiencies in power transfer (coupling losses) engendered in having energy sources and sinks of differing impedance characteristics are well known in electronics. This phenomena asserts itself in optics as well. The chief contributors to coupling losses are area and numerical aperture mismatches of source and sink. Area losses may be minimized by accurate alignment of source to sink and by ensuring that the area illuminated by the source is small with respect to the sink angle of acceptance Numerical aperture losses may be minimized by ensuring that the source angle of emission is narrower than the angle of acceptance of the receiver.
Reflection and refraction are simultaneously the guiding principles and nemeses of fiber optic designs. The key element is the difference in characteristic impedance between media of propagation. The impedance of free space (Z.sub.s) is approximately 377 ohms. The characteristic impedance of an isotropic dielectric medium (Z.sub.m) is given by the following equation: ##EQU1## where n is the index of refraction of the medium. Reflections where detrimental may be controlled by use of impedance transforming sections (anti-reflection coatings) between media. Refractions where detrimental may be minimized by careful attention to the geometries of the desired radiation paths.
Current commercial fiber-optic communications utilize light emitting diodes (LEDs) which are broadband (40-60 nanometers(nm)) optical sources gated in the time domain to effect transmission of information. Techniques have been developed to split these bands through filtering or diffraction to allow use of the frequency domain as well. The use of the frequency domain within optical fibers has proven impractical for commercial purposes due to the severe attenuation of optical power by state of the art frequency and fiber coupling mechanisms. For example, in U.S. Pat. No. 3,953,727, a system for transmitting independent communication channels through a light-wave medium, high attenuation of the signal strengths results through multiple filtrations, reflections, and coupling inefficiencies of the fiber. Short of incorporating the wavelength selective dielectric films onto the structure of the LED, little gain has thus far been achieved in the use of discrete filter assemblies as discussed in that patent.
The geometries of the multiplexing sections, however, offer many possibilities for improvement. Multi-channel and byte-wide data transfer without complicated serialization and de-serialization would provide immediate benefits. Combinations of this frequency division multiplexing and time division multiplexing could expand the present limitations on data security. Multi-channelled and full duplex communications over a single fiber may be possible.
Accordingly, an object of this invention is to transfer byte-wide data streams through a single optic fiber.
A further object of the invention is to minimize coupling inefficiencies between LEDs and fibers.
Another object of the invention is to minimize coupling inefficiencies between LEDs and fibers and the simplification and reduction of the components used to achieved this end.
Yet another object of the invention is the enhancement of data security, regardless of whether the transfer of byte-wide data is in parallel data or independent serial data streams, since tapping of multi-wavelength fibers is easily detected by monitoring of the variation of boundary wavelength signal levels at the detector.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.